I am not able to use texture paint (2.8)
Make sure you have an image open in the Image Editor.

Pressing the small picture icon will allow you to select existing images in your project. New will create a new blank image, and Open will allow you to append an image from your computer into the Blend project.



If there are missing textures from your project (those show up as pink), you can tell Blender which folders to look for those textures in by going to File > External Data > Find Missing Files